Swiveling wheel assembly for a cart

ABSTRACT

A swivelling wheel assembly containing a wheel whose swivelling action around a vertical axle may be locked or unlocked by rotating the wheel in one direction or the other. A curved double control lever is housed in a space between the thrust bearing and the tread of the wheel in the area of the lateral extensions of the fork joint. A lower lever arm holds a friction device engaging the tread of the wheel. An upper lever arm contains a locking means which can be received in a locking slot constructed in a housing component which is fixed relative to the pivot axle. A circular cover of the fork joint contains a tracking groove to guide the upper lever arm.

BACKGROUND OF THE INVENTION

This invention relates to the wheel assembly for a cart or other mobilemechanism, of the type which contains a wheel which swivels around avertical axle, and may be locked or unlocked by the rotation of thelatter in one direction or the other. Such wheel assemblies include athrust bearing with at least one circular housing fixed by axle, a forkjoint with at least two lateral extensions whose lower ends support thewheel's horizontal axle, and one upper circular cover connected to theupper ends of the lateral extensions. The cover cooperates with bearingsplaced around the lower end of the vertical axle so that the fork jointcan rotate around the latter. Prior art wheel assemblies are known tocontain devices for locking the fork joint in relation to the verticalswivel axle. Such devices are disclosed, for example, in British PatentGB-A-2106-378.

In the device disclosed in the '378 patent, part of the fork joint'slocking device is accessible from the outside and may be damaged.Because of this, the proper operation of the device cannot be assured onan ongoing basis. Moreover, the known wheel assembly of this patentrequires a major modification to the fork joint, particularly in thearea of the wheel axle, which must be capable of moving in the oblongopenings made in the fork joint. Another problem with this knownswivelling wheel is the fact that controlling the unlocking of the forkjoint entails abruptly stopping the wheel for an instant. In addition,the locking devices for the known swivelling wheel assembly have a largenumber of unprotected individual parts, and are therefore, subject torapid wear and deterioration.

SUMMARY OF THE INVENTION

The purpose of the present invention is to eliminate such problems andto provide a swivelling wheel which is simple in design, protectedagainst external damage, and has a reduced number of individual parts.

Within the framework of a swivelling wheel assembly of the type referredto above, the assembly of the present invention includes a doublecontrol lever which is pivotally mounted in the space between theexternal housing of the fork joint and the wheel's tread in the area ofthe fork joint's lateral extensions. The lever is in the form of acurved lever whose two arms encompass an angle of between 90° and 160°.The lower arm moves between two outermost positions located in proximityto a radial plane running through the pivot axis of the lever and thewheel's axis of rotation. The lower lever arm has at its lower end afriction device which is resiliently urged into engagement with thewheel's tread. The upper lever arm is housed in the space enclosed bythe fork joint above the wheel's tread, and includes, as a locking meansa tip which extends perpendicularly to the lever's pivot axle, and inthe direction toward the periphery of the fork joint. A second lockingmeans consists of a locking slot for receiving the locking tip withoutany significant lateral play. The slot is constructed in the lower endof the vertical swivel axle or a component which is an integral part ofthe latter. One of the two stops for limiting the pivoting movement ofthe control lever is placed in the space located about the wheel andbetween the lateral extensions of the fork joint. This stop works inconjunction with a first lower portion of the control lever. The secondstop consists of either the bottom of the locking slot, or the lowerbottom edge of the component containing the locking slot, and works inconjunction with the upper lever arm's locking tip.

A circular cover of the fork joint contains a tracking groove extendingalong the center plane of the fork joint and perpendicular to both thewheel axle and the control lever's pivot axle. The tracking grooveoperates to guide, without significant lateral play, a portion of theupper lever arm or the locking tip along its entire path between the twooutermost positions of the lever.

With the construction of the present invention, the locking device iscompletely protected, contains a reduced number of individual parts, andis continually operational, despite possible wear on the part of thewheel's tread.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation, partly in cross-section, of a firstembodiment of the swivelling wheel assembly of the present invention;

FIG. 2 is a perspective view of one part of the locking device of thepresent invention;

FIG. 3 is an exploded perspective view of the several components whichmake up the second part of the locking device;

FIG. 4 is a side elevation, partly in cross-section, of a secondembodiment of the swivelling wheel assembly of the present invention;

FIG. 5 is a side elevation, partly in cross-section, of a thirdembodiment of the swivelling wheel assembly of the present invention;and

FIG. 6 is a side elevation, partly in cross-section, of a fourthembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown on the attached drawings, a cart or other mobile platform,which is not shown, contains a wheel 2 which is capable of swivellingaround the vertical swivel axle 3 and of being locked or unlocked by therotation of the wheel 2 in one direction or the other. The wheel 2contains a horizontal rotating axle 4 whose ends are supported by atleast two lateral extensions or forks 5 of a fork joint 6. The upper endof the fork joint, which is in the form of a circular cover 7, isfastened by means of a thrust bearing 8 to the lower end of the verticalswivel axle 3. It may be noted that the vertical swivel axle islaterally offset in relation to the vertical plane passing through thehorizontal axle 4 of the wheel 2.

As can be seen on FIGS. 1 and 4, the thrust bearing 8 includes a firstcircular housing 9 and a second circular housing 10 fixedly mounted,like the first, on the lower end of the vertical axle 3. The twohousings 9 and 10 are open at the bottom. The first housing 9 is anexternal housing and is located above the second lower housing 10 andhas an external rounded extension 9a and a circular downwardly turnedbase portion 9b which, in the area of the internal opening, rests on thecircular base 10a of the lower housing 10. The two housings 9, 10 aremounted on the vertical axle 3 and their inner edges are enclosed by tworing-shaped collars 3a, 3b of the vertical axle 3.

At its lower end, the lower housing 10 has an outer circular edge 10bwhose upper surface serves as the rolling path for a first series ofroller bearings or similar devices 11 which support the circular cover 7of the fork joint 6. In the area of the circular junction between therounded extension 9a and the circular base portion 9b, the upper housing9 defines a rolling path in the form of a groove 12 which houses asecond series of roller bearings 13 resting on the upper side of thecircular cover 7.

As shown in FIGS. 1 and 4, the external housing 9 covers and surrounds,particularly on its lower side, the circular cover 7 and the lowerhousing 10. The lower housing 10 is, in turn, enclosed by the cover 7.

It is possible to use an upper housing 9 whose extension 9a is curvedupwards from the outer edge of the groove 12. In such a construction,which is shown in FIG. 6, the housing 9 is open toward the bottom inorder to have a circular external groove 14 adjacent to the groove 12.The groove 14 is open toward the top and is slightly deeper than thegroove 12. The groove 14 is bounded by a horizontal flange 15 which isslightly raised and can be used, for example, to fasten the bearing 8 tothe connecting receptacle of a cart which is not shown.

In the embodiment shown in FIG. 5, the thrust bearing 8 carries tworoller flanges 8a, 8b. The second roller flange 8b encloses the firstflange 8a and is fixed to the circular cover 7 for the fork joint 6. Thefirst flange 8a is, in turn, fixedly mounted on the vertical axle 3 andthe external housing 9.

The swivelling wheel 1 also contains a locking device 16 for the forkjoint 6. The locking device includes two parts. One part is mounted onthe fork joint while the other part is fixed relative to the verticalaxle 3. The part of the locking device on the fork joint includes apivot mechanism 17 in the form of a double curved control lever whichhas an upper lever arm 17a and a lower lever arm 17b, which encompass anangle of between 90° and 160°. This double curved control lever 17 ishoused in the space between the thrust bearing 8, the lateral extensions5 of the fork joint 6 and the wheel 2. In the examples shown in FIGS. 1,4 and 5, the lever is located vertically between the upper or externalhousing 9 of the bearing 8 and the tread 2a of the wheel 2, andlaterally between the lateral extensions 5 of the fork joint 6. Thejoining piece between the two lever arms 17a, 17b is connected to thelateral extensions 5 of the fork joint 6 using a single horizontal axle18 or two coaxial pivot axles 18 riveted onto the extensions 5. Thepivot axle 18 of the lever 17 is located on the side of the fork joint 6on which the vertical distance between the locking device 16 and thetread 2a of the wheel 2 is the greatest.

The locking device 16 also includes a friction device 19 installed onthe lower end of the lower lever arm 17b and permanently and resilientlyapplied to the tread 2a of the wheel 2. This friction device is capableof causing the double control lever 17 to pivot toward one of twooutermost positions about the pivot axle 18. In one such position, thesupporting fork joint is immobilized in relation to the vertical axle 3;and in the other, the fork joint 6 is free to rotate in relation to thevertical axle 3.

The friction device 19 consists of a tubular piston 19a which is guidedso that it can slide axially in a housing 20 with an appropriatesection, for example, a circular section, constructed coaxially in thelower lever arm 17b in such a manner as to be open at the bottom. Thelower end 19b of the tubular piston projects from the housing 20 in thelower lever arm 17b, and is shaped as a spherical cap. A spring 21 iscontained in the lower lever arm 17b and pushes the friction device 19against the tread 2a of the wheel 2. This spring 21 is placed partiallyinside the tubular piston 19a and partially outside the piston in thehousing 20 of the lower lever arm 17a. The upper end of the springengages against the pivot axle 18. Alternatively, the upper end of thespring can engage against an internal flange in the housing 20 or otherinternal structure of the double control lever 17. At its other lowerend the spring engages against the friction device 19, and, moreprecisely against the internal surface of the spherical cap 19b.

The locking device 16 also includes a locking means 22 consisting of atip which extends from and is an integral part of the free end of theupper lever arm 17a. The locking means 22 extends perpendicularly to thepivot axle 18 of the lever 17 in the direction toward the periphery ofthe thrust bearing.

The remaining part of the locking device 16 includes a second lockingmeans 23 which is located on a component of the wheel assembly whichremains stationary during rotation of the fork joint 6 about the axle 3.The locking means 23 operates in conjunction with the locking means 22to immobilize the supporting fork joint 6 against rotation on itsvertical swivel axle 3. As shown in FIGS. 1 and 6, the locking means 23is located in the inner, lower housing 10 of the thrust bearing 8.Alternatively, in the embodiments of FIGS. 4 and 5, the locking means 23is an integral part of the external stationary housing 9 of the bearing8.

The locking means 23 consists of a locking slot open in the direction ofthe first locking means 22 and capable of receiving, without anysignificant lateral play, the first locking means, that is to say, thelocking tip 22 which is an integral part of the upper lever arm 17a.This slot 23 is located along a vertical center plane of the fork joint6. This plane passes through the axis of the vertical axle 3 and isperpendicular to the horizontal axis 4 of the wheel 2 and to thehorizontal pivot axle 18 of the double lever 17 when the cart on whichthe swivelling wheel assembly 1 is mounted moves straight ahead. As canbe seen in the drawing, the pivot axle 18 of the double lever 17 islocated near the periphery of the circular cover 7 of the fork joint 6and also near the locking slot 23.

Finally, the locking devices 16 include two stops 24, 25 which serve tolimit the pivot angle of the double control lever 17 between its twooutermost positions. These two stops form with the pivot axle 18 arelatively small angle on the order of 3° to 15°, and are located inproximity to a radial plane of the wheel 2. The latter plane passesthrough the lever's pivot axle 18 and the wheel's axle 4 of rotation. Inthis manner, the lower lever arm 17b shall not deviate overly from aposition in which its axis extends vertically in relation to the tread2a, i.e., perpendicular to the surface of the tread where engagementwith the friction device 19 occurs.

The first stop 24 for the control lever 17 is placed in the spacelocated above the wheel 2 and between the lateral extensions 5 of thefork joint 6 and works in conjunction with a first portion, for example,the lower lever arm 17b of the control lever 17, while the second stopconsists of the downwardly facing bottom surface of the locking slot 23and the downwardly facing bottom edge surface of the housing 9 or 10which contains the slot 23. The stop 25 works in conjunction with thelocking tip 22 to limit the pivoting of the lever 17 in thecounter-clockwise direction as viewed in the drawings.

The circular cover 7 of the fork joint 6 contains a tracking groove 26along the center vertical plane of the fork joint 6. This groove lies ina plane perpendicular to the axle 4 of the wheel 2 and the pivot axle 18of the double lever 17; and, as with the locking device 16, is locatedon the side of the fork joint 6 on which the distance between the cover7 and the tread 2a is the greatest. The tracking groove 26 is open atthe bottom and 25 aligned with the upper lever arm 17a and locking tip22 so as to guide, with no significant lateral play, these members intheir pivoting path between the control lever's 17 outermost positions.Due to this construction, the locking of the fork joint 6 is transmittedto the swivel axle 3 along the shortest distance between the swivellingfork joint 6 and housing 9 and 10 which is fixedly mounted on thevertical swivel axle 3.

The locking slot 23 may be placed radially outwardly of the trackinggroove 26 formed in the cover 7 as in the construction of FIGS. 4 and 5where the slot 23 is located on the external housing 9. The locking slot23 may also be placed radially inwardly of the groove 26 as in theconstruction shown in FIGS. 1 and 6 where the slot is placed on theinternal housing 10.

We claim:
 1. In a swivelling wheel assembly (1) containing a wheel (2)whose swivelling action around a vertical swivel axle (3) having a lowerend may be locked or unlocked by rotating the wheel (2) in one directionor the other, respectively, the assembly including a fork joint (6)having an upper end (7) and a lower end with the upper end (7) mountedon the lower end of the vertical swivel axle (3), for selectivelyswivelling action around said vertical axle (3), a horizontal axle (4)fixed to the lower end of the fork joint for mounting the wheel (2) forrotation about a horizontal axle which is fixed relative to the forkjoint, and a locking device (16) for locking the fork joint (6) inrelation to the vertical swivel axle (3), the improvement wherein: thelocking device (16) comprisesa) a first locking part (23) fixed relativeto said vertical axle (3); and b) a second locking part mounted on thefork joint (6), said second locking part including:i) a control means(17) having two ends (19, 22), said control means being mounted on thefork joint for movement between a first position and a second position,one end (19) of said ends extending downwardly engaging against thewheel (2), and the other end (22) being positioned adjacent said firstlocking part (23) for movement between a first wheel locking positionengaging with said first locking part (23) and a second wheel unlockingposition spaced from said first locking part (23) upon movement of saidcontrol means between said first position and said second position, andii) means (21) for maintaining the one end (19) of the control means(17) in driven engagement with said wheel for limited movement with thewheel between two positions as the wheel rotates in opposite directionswhereby the rotation of the wheel in one direction moves the one end(19) to one position and the second end (22) to said first wheel lockingposition and rotation of the wheel in a second opposite direction movesthe one end (19) to a second position and the second end (22) to saidsecond wheel unlocking position.
 2. In a swivelling wheel assembly (1)containing a wheel (2) whose swivelling action around a vertical axle(3), having a lower end, may be locked or unlocked by rotating the wheel(2) in one direction or the other, respectively, the assembly includinga bearing (8), at least one housing (9, 10) fixedly mounted to the lowerend of the vertical swivel axle (3), a fork joint (6) having two lateralextensions (5) whose lower ends support a horizontal axle (4) for thewheel (2) and whose upper end includes a cover (7) which cooperates withthe bearing (8) to permit rotation of the fork joint around the axle(3), and a locking device (16) for locking the fork joint (6) inrelation to the vertical swivel axle (3), the improvement wherein:thelocking device (16) comprises: a) a first locking part (23) fixedrelative to said vertical axle (3) and located within the outer lateralperiphery of the housing (9); and b) a second locking part locatedvertically between the bearing (8) and the wheel (2) and laterallybetween the extensions (5) of the fork joint (6) so as to be protectedagainst damage by contact therewith by objects externally of the wheelassembly, said second locking part including:i) a control means (17)having two ends (19, 22), said control means being mounted between theextensions (5) of the fork joint (6) for movement between a firstposition and a second position, one end (19) of said ends extendingdownwardly engaging against the wheel (2), and the other end (22) beingpositioned adjacent said bearing (8) and the lower end of the verticalaxle (3) for movement between a first wheel locking position engagingwith said first locking part (23) and a second wheel unlocking positionspaced from said first locking part (23) upon movement of said controlmeans between said first position and said second position, and ii)means (21) for maintaining the one end (19) of the control means (17) indriven engagement with said wheel for limited movement with the wheelbetween two positions as the wheel rotates in opposite directionswhereby the rotation of the wheel in one direction moves the one end(19) to one position and the second end (22) to said first wheel lockingposition and rotation of the wheel in a second opposite direction movesthe one end (19) to a second position and the second end (22) to saidsecond wheel unlocking position.
 3. In a swivelling wheel assembly (1)containing a wheel (2) whose swivelling action around a vertical axle(3) may be locked or unlocked by rotating the wheel (2) in one directionor the other, the assembly including a thrust bearing (8) which has atleast one housing (9, 10) fixedly mounted to the lower end of thevertical swivel axle (3), a fork joint (6) having two lateral extensions(5) whose lower ends support a horizontal axle (4) for the wheel (2) andwhose upper end includes a cover (7) which cooperates with the bearing(8) to permit rotation of the fork joint around the axle (3) and alocking device (16) for locking the fork joint (6) in relation to thevertical swivel axle (3), the improvement wherein:the locking device(16) comprises: a) a first locking part (23) fixed relative to saidvertical axle (3) and located within the outer lateral periphery of thehousing (9); and b) a second locking part located vertically between thethrust bearing (8) and the wheel (2) and laterally between theextensions (5) of the fork joint (6) so as to be protected againstdamage by contact therewith by objects externally of the wheel assembly,said second locking part including:i) a double control lever (17) havingtwo ends (19, 22), said control lever being pivotally mountedintermediate its ends on an axle (18) located between the extensions (5)of the fork joint (6) for pivoting movement between a first outermostposition and a second opposite outermost position, one end (19) of saidends extending downwardly engaging against the wheel (2), and the otherend (22) being positioned adjacent said thrust bearing (8) and the lowerend of the vertical axle (3) for movement between a first wheel lockingposition engaging with said first locking part (23) and a second wheelunlocking position spaced from said first locking part (23) uponpivoting of said control lever between said first outermost position andsaid second outermost position, and ii) means (21) for maintaining theone end (19) of the lever (17) in driven engagement with said wheel forlimited movement with the wheel between the two positions as the wheelrotates in opposite directions whereby the rotation of the wheel in onedirection moves the one end (19) to one position and the second end (22)to said first wheel locking position and rotation of the wheel in asecond opposite direction moves the one end (19) to a second positionand the second end (22) to said second wheel unlocking position.
 4. Inthe swivelling wheel assembly in accordance with claim 3, theimprovement wherein:a) at least one of said housings (9, 10) is circularin lateral dimension as measured around said vertical axle (3) andincludes a bottom circular peripheral surface facing the other end (22)of said control lever (17) and in vertical axial alignment therewiththroughout the swivelling action of said wheel assembly around saidvertical axle (3); b) said first locking part (23) is defined by avertically extending slot (23) in the housing (9, 10) extending upwardlyfrom said bottom circular peripheral surface to a bottom surface; and c)the other end (22) of the control lever (17) engages within said slot(23) when in said first wheel locking position.
 5. In the swivellingwheel assembly in accordance with claim 4, the improvement wherein:a)the cover (7) of the fork joint (6) is circular in lateral dimension asmeasured around said vertical axle (3) and includes a verticallyextending tracking groove (26); and b) said other end (22) of saidcontrol lever is rotatably fixed relative to said cover and verticallyslidable in said tracking groove (26) as the control lever pivots aboutsaid axle (18).
 6. In the swivelling wheel assembly in accordance withclaim 5, the improvement wherein:a) the bearing (8) includes an outercircular housing (9) and an inner circular housing (10) with the outerhousing (9) circumferentially surrounding and vertically covering theinner housing (10); b) the cover (7) is located between said inner andouter housing (9, 10); and c) the vertically extending slot is formed inthe inner housing (10).
 7. In the swivelling wheel assembly inaccordance with claim 5, the improvement wherein:a) the bearing (8)includes an outer circular housing (9) and an inner circular housing(10) with the outer housing (9) circumferentially surrounding andvertically covering the inner housing (10); b) the cover (7) is locatedbetween said inner and outer housings (9, 10); and c) the verticallyextending slot (23) is formed in the outer housing (9).
 8. In theswivelling wheel assembly in accordance with claim 4, the improvementfurther comprising:a) first and second means (24, 25) for limiting thepivoting movement of the control lever (17) between said first andsecond outermost positions;i) said first stop means including a stopmember extending between the lateral extensions (5) of the fork joint(6) and in the path of pivoting movement of a portion of the leverlocated below said axle (18), and b) said second stop means is definedby the lower surface of said housing (9, 10) and the bottom surface ofsaid vertically extending slot (23).
 9. In the swivelling wheel assemblyin accordance with claim 8, the improvement wherein:a) said verticallyextending slot (23) is aligned with the direction of linear movement ofthe wheel upon rotation thereof in said one direction.
 10. In theswivelling wheel assembly in accordance with claim 9, the improvementwherein:a) the fork joint (6) is mounted on the vertical axle (3) forswivelling movement to a first position with the axle (4) of the wheel(2) located at a position behind the axle (3), as measured in thedirection of linear movement of the wheel (2), when said wheel isrotated in said one direction.
 11. In the swivelling wheel assembly inaccordance with claim 10, the improvement wherein:a) the vertical slot(23) and the control lever (17) are located at a position forwardly ofthe vertical axle (3), as measured in the direction of linear movementof the wheel (2), when the wheel is rotated in said one direction andthe fork joint is swivelled to said first position.
 12. Swivelling wheelassembly (1) containing a wheel (2) whose swivelling action around avertical axle (3) may be locked or unlocked by rotating the wheel (2)along its thread (2a) in one direction or the other, respectively, saidassembly comprising a thrust bearing (8) which has at least uppercircular housing (9) which is fixedly mounted, by its center section, tothe lower end of the vertical swivel axle (3), a fork joint (6) having acenter plane and including at least two lateral extensions (5) whoselower end support a horizontal axle (4), extending perpendicular to saidcenter plane, for the wheel (2), an upper circular cover (7) connectedto the upper ends of the lateral extensions )5) and connected to thethrust bearing at the lower end of the vertical axle (3) for rotationabout said axle, a locking device (16) for locking the fork joint (6) inrelation to the vertical swivel axle (3); said locking deviceincluding:a) a double control lever (17) housed in the space enclosed bythe lateral extensions (5) of the fork joint (6), the bearing (8) andthe periphery of the wheel (2) and having an upper lever arm (17a) witha free end and a lower lever arm (17b) encompassing an angle of between90° and 160°, said lever (17) including: b) a horizontal axle (18)extending between said extensions (5) pivotally mounting said leverbetween the periphery of the wheel (2) and the thrust bearing (8) withthe lower arm (17b) moving between two outermost positions located inthe proximity of a radial plane passing through and parallel to thepivot axle (18) of the cover lever (17) and the swivel axle (3) of thewheel (2); c) the lower arm (17b) comprises a friction device (19)resiliently urged into frictional engagement against a portion of awheel (2) and capable of causing the control lever (17) to pivot betweensaid two outermost positions, in one of which the fork joint (6) isstopped from rotating about the vertical axle (3), and in the other ofwhich, the fork joint (6) is free to rotate about the vertical axle (3);d) a first locking means (23) fixed relative to the vertical axle (3)and a second locking means (22) integral with the free end of the upperlever arm (17a) and in operative position relative to the first lockingmeans (22) for immobilizing the fork joint (6) on its vertical axle (3),said first locking means (23) extending perpendicular to the pivot axle(18) of the lever (17) and toward the periphery of the thrust bearing(8) and including a locking slot (23) capable of receiving the secondlocking means (22) within significant lateral play; e) two stops (25,25), at least one of which is connected to the fork joint (6) to limitthe pivot angle of the control lever (17) between its two outermostpositions; f) said one stop being located in the space located above thewheel (2) and between the lateral extensions (5) of the fork joint (6)and cooperating with a first portion (17b) of the control lever (17) andthe second stop (25) defined by the bottom of the locking slot (23) andthe lower bottom of at least one housing (9, 10) for cooperating withthe second locking means (22) of the upper lever arm (17a); and g) atracking groove (26) in the circular cover (7) of the fork joint (6),said groove extending along the center plane of the fork joint,perpendicular to the axle (4) of the wheel (2) and the pivot axle (18)of the control lever (17), said tracking groove (26) being open at thebottom and receiving, without significant lateral play, a portion of theupper lever arm (17a) and the second locking means (22) throughout itspath between the two outermost positions of said control lever. 13.Swivelling wheel assembly in accordance with claim 12, wherein thehousing (9) for the thrust bearing (8) contains a rounded extension (9a)whose lower edge covers and encloses the cover (7) and upper end of thefork joint (6).
 14. Swivelling wheel assembly in accordance with claim12, wherein the friction device (19) is continually urged against thetread (2a) of the wheel (2) by the action of a spring (21) contained inthe lower lever arm (17b) and guided axially in relation thereto. 15.Swivelling wheel assembly in accordance with claim 14, wherein the lowerlever arm (17b) includes housing (20) which is open at the bottom andslideably receives said friction device (19) and spring (21). 16.Swivelling wheel assembly in accordance with claim 15, wherein thehousing (20) extends upwardly from the open bottom of the lower leverarm (17b) to the pivot axle (18) and one end of the spring (21) engageson the pivot axle (18) and the other end engages against the frictiondevice (19).
 17. Swivelling wheel assembly in accordance with claim 16,wherein the friction device (19) is a tubular piston (19a) which slidesinto the housing (20) and whose lower end projects from the lower leverarm (17b) and is enclosed by a spherical cap and the other end of thespring (21) is housed within the tubular piston (19a).
 18. Swivellingwheel assembly in accordance with claim 12, wherein the thrust bearing(8) includes a lower circular housing (10) having an open bottom andwhich is located within and under the first upper circular housing (9)and is fixedly mounted on the lower end of the vertical swivel axle (3),and the circular cover (7) of the fork joint (6) is located between twohorizontal series of roller bearings (11, 13) so that it can rotatearound the vertical swivel axle (3).
 19. Swivelling wheel assembly inaccordance with claim 12, wherein the thrust bearing (8) has first andsecond roller flanges (8a, 8b), of which the second roller flange (8b)engages the first flange (8a) and is an integral part of the circularcover (7) of the fork joint (6), and the first flange (8a) is anintegral part of the vertical axle (3) and of the upper circular housing(9).
 20. Swivelling wheel assembly in accordance with claim 18, whereinthe locking slot (23) is constructed in a lower surface of the lowerhousing (10) of the thrust bearing (8) and the second locking means (22)on the upper lever arm (17a) is located within the circumferentialperiphery of the cover (7) of the fork joint (6).
 21. Swivelling wheelassembly in accordance with claim 12, wherein the locking slot (23) isconstructed in a lower surface of the upper circular housing (9) of thethrust bearing (8) and the locking means (22) on the upper lever arm(17a) is located substantially within the circumferential periphery ofthe upper housing (9) of the thrust bearing (8).
 22. Swivelling wheelassembly in accordance with claim 12, wherein the first stop (24)consists of a transverse rod extending parallel to the pivot axle (18)of the double lever (17) between the extensions (5) of the fork joint(6) and mounted on the latter for engagement with the lower lever arm(17b).